Satellites in geostationary orbits (GSOs) have been widely preferred for several decades because of the economic advantages afforded by such orbits. In a geostationary orbit, a satellite traveling above the Earth's equator, in the same direction as that in which the Earth is rotating, and at the same angular velocity, appears stationary relative to a point on the Earth. These satellites are always "in view" at all locations within their service areas, so their utilization efficiency is effectively 100 percent. Antennas at Earth ground stations need be aimed at a GSO satellite only once; no tracking system is required.
Coordination between GSO's and with terrestrial services is facilitated by governmental allocation of designated "slots" angularly spaced according to service type. Given the desirability of geostationary satellite orbits and the fact that there are only a finite number of available "slots" in the geostationary "belt," the latter capacity has been essentially saturated with satellites operating in desirable frequency bands up through the Ku-band (up to 18 GHz). As a result, the government has been auctioning the increasingly scarce remaining slots.
This has encouraged the development of complex and expensive new systems including those using low Earth orbits (LEO's), medium Earth orbits (MEO's), and/or higher frequencies, for example, the Ka band (up to approximately 40 GHz). Growth to higher frequencies is limited by problems of technology and propagation, and expansion in satellite applications requires exploitation of the spatial dimension (i.e., above and below the GSO belt). A host of proposed LEO and MEO systems exemplify this direction. A drawback of LEO and MEO systems for users is the relative uncertainty of satellite position, and rapid motion, leading typically to the use of omnidirectional antennas having low gain, which limits data rate.
Highly elliptical orbits (HEO) such as the 12-hour "Molniya" long used by Russia, and the European Space Agency's 8-hour "Archimedes" have been used. HEO's disadvantages include a shorter fraction of service to a given area (fractionally geosynchronous period causes multiple nodes over the earth) and require specific 63.degree. inclination (to minimize fuel requirements due to low perigee). LEO, MEO, and HEO systems require more satellites for coverage at a specified elevation angle to a single service area than does the present invention.
Another apparent drawback to the use of all inclined orbits is that of relative movement with respect to the ground. For wide bandwidths, two-dimensional tracking ground station antennas would be required. Tracking antennas are relatively expensive and thus are not considered for consumer applications.
There has been no known prior effort to exploit overhead systems of inclined eccentric geosynchronous orbits (IEGOs) in a systematic manner, even though the unused domain of inclined eccentric geosynchronous orbits offers great potential for the coordinatable growth of satellite service.
While the various prior systems function relatively satisfactorily and efficiently, none discloses the advantages of the overhead system of inclined, eccentric geosynchronous satellite orbits in accordance with the present invention as is hereinafter more fully described.